As machines become more complex and valuable, there is a greater need to protect them, and the systems they support, from the consequences of breakdown. This is relevant in the semiconductor industry, for example, where machine failure can contribute to the loss of a very valuable batch of wafers. Dry vacuum pumps have been successfully used in the semiconductor industry. Nonetheless, the harsh nature in many semiconductor processes creates a challenge for condition monitoring of these systems (see Troup et al. (1988), Dry pumps operating under harsh conditions in the semiconductor industry, Journal of Vacuum Sci. Technol, Vol. 7, 2381-2386). Any diagnostic scheme should be able to operate under all conditions and be able to detect faults, creating an alarm to warn the user that maintenance must be conveniently scheduled before catastrophic loss occurs.
Sliding mode techniques have been used widely for fault detection schemes in recent years. Their main advantage is that they exhibit fundamental robustness against certain kinds of parameter variations. The design procedure is characterised by two phases: selection of an appropriate surface where the system will demonstrate desired dynamics and selection of an injection signal that will force the system to reach and maintain its sliding motion.
A number of engineering and biomedical applications have used sliding mode observers in order to recreate fault signals. Examples can be found in Jones et al. (2000), Aspects of diagnostic schemes for biomedical and engineering systems, IEE Proc.-Sci. Meas. Technol, Vol. 147, No. 6. A particular sliding mode observer for fault detection and isolation is described by Edwards et al. (1999), in Sliding mode observers for fault detection and isolation, Automatica, Vol. 36, 541-553. The novelty of the approach is that the observer attempts to reconstruct the fault signals rather than detect the presence of a fault through a residual signal. The proposed observer is designed to maintain the sliding motion, even in the presence of faults, which are detected by analysing the so-called equivalent output injection signal obtained from the discontinuous injection signal required to maintain sliding. The equivalent injection signal is thus not the injection signal applied to the observer but represents the injection required, on average, to maintain sliding motion. The equivalent injection signal can be readily obtained from by appropriate filtering of the applied, usually discontinuous, injection signal required. An alternative sliding mode observer scheme for monitoring is described in Hermans et al (1996), Sliding mode observer for robust sensor monitoring, Proceedings of the 13th IFAC World congress, pp. 211-216. In that document, a design approach is adopted whereby, when a fault occurs, the observer is disturbed from its surface and sliding ceases. However, the sliding mode control theory indicates such an approach is difficult to implement since the observer is designed in such a way in order to keep the system always in sliding motion. In addition, the choice of gain to maintain sliding motion from the theory is often conservative. Therefore, it is difficult to ensure a fault induces a break in sliding.
Many real, reliable sensors are not commercially or technically viable in corrosive, toxic, or high-temperature environments within rotating machines (e.g. inside a semiconductor process vacuum pump).